This first screenshot is made specifically for the Rift (sorry for low quality, I'm on an old laptop).
It has been lens-corrected (you can tell because the images have been warped into "circles with corners", but sometimes these can just be plain circles),
and it has chromatic aberration correction (you can tell from the color fringing when viewing without the Rift).
Crossing your eyes to view this will give give you a crisp image but it's just not right:

If we take away the lens-correction and chromatic aberration correction, this is what you get. Better, but the 3D is still wrong:

Finally, if we swap the left and right images (which can either be done by cropping and moving, or mirroring, or by using a negative IPD in the Rift code),
we get a stereo-pair that is perfect for cross-eyed viewing: